A wide variety of perishable items particularly such as food products are routinely shipped in a chilled or frozen state to avoid product spoilage and/or contamination. In this regard, refrigeration systems for use in transport or shipping vehicles such as rail cars, trucks, and the like are well known and have been widely used for many years. In one common form, such refrigeration systems have utilized conventional mechanical refrigeration units for maintaining a controlled temperature within an insulated storage container or compartment. However, mechanical refrigeration units are relatively costly and require significant maintenance over a typical service life. Moreover, mechanical refrigeration equipment normally exhibits relatively high power or energy consumption, wherein an appropriate fuel or electrical power source for the refrigeration unit must be carried with the shipping vehicle.
In recent years, non-mechanical refrigeration systems have acquired significant commercial interest in efforts to avoid the cost and related complexities associated with standard mechanical refrigeration equipment. In a non-mechanical system, the rail car or the like is modified to carry an expendable supply of a cryogenic material, particularly such as carbon dioxide in solid form, commonly known as dry ice. The cryogenic material is supported in close association with the items being shipped to maintain a sufficiently low temperature to prevent spoilage. Over a period of time which may include several days, the solid carbon dioxide sublimates into the form of cryogenic gas which is normally circulated over or about the perishable items prior to exhausting to the atmosphere through suitable vents.
The traditional method to transport frozen food in shipping containers is to manually layer the food items with slices of dry ice in a cardboard or insulated box on a pallet. This is labor-intensive and inefficient because the food is in direct contact with the CO2 ice or snow, causing the food to freeze below the required temperature and the CO2 to sublime faster. This is an inefficient use of CO2 because the food is in direct contact and causes it to sublime faster, thus reducing the time the shipping container will be refrigerated.